1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to more precisely detecting contact between a recording head and a magnetic disk in a magnetic disk drive system.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more recording heads (sometimes referred to as sliders) that include read elements and write elements. An actuator/suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to fly a particular height above the magnetic disk. The height to which the recording head flies depends on the shape of the ABS. As the recording head flies on the air bearing, a voice coil motor (VCM) moves the actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
The magnetic disk includes data regions and servo regions. The servo regions are used to provide sector information, timing information, positioning information, etc. For example, as the magnetic disk makes a revolution, the read element passes over burst fields in the servo regions. The signal read from the burst fields may be used to generate a timing signal. The signal read from the burst fields may also be used to generate a quadrature signal that is used for centering the read element and write element over the center of a track. The information read from the servo regions may be generally referred to as servo data. The servo data is feed back to a control system, which controls the VCM, controls the rotational speed of the magnetic disk, etc.
One factor that contributes to the effective reading and writing by the recording head is the spacing of the read/write elements in relation to the surface of the magnetic disk. The spacing between the read/write elements generally depends on the fly height of the recording head, which is determined by the air bearing surface (ABS) of the recording head. As areal densities of magnetic disk increase, it becomes more important to precisely control spacing of the read/write elements in relation to the magnetic disk, as the spacing may be 10 nanometers or less.
To further control the spacing between the read/write elements and the magnetic disk, some recording heads include heating elements that are fabricated in the recording head proximate to the read/write elements. The read/write elements are fabricated from materials that have a different thermal rate of expansion than the body of the recording head. Thus, when a current is applied to the heating elements, the read/write elements protrude from the ABS of the recording head. Thus, the protrusion causes the read/write elements to extend toward the surface of the magnetic disk, which reduces the spacing between the read/write elements and the magnetic disk. The use of heating elements (sometimes referred to as Thermal Fly-Height Control) allows for more precise spacing between the read/write elements and the magnetic disk.
To calibrate a magnetic disk drive system to have a desired spacing between the read/write elements and the magnetic disk, the control system applies a motor current to a spindle motor which in turn rotates a spindle connected to the magnetic disk. As the magnetic disk rotates, the recording head flies over the surface of the magnetic disk on the air bearing. The control system then incrementally increases the power applied to the heating elements in order to increase the protrusion of the read/write elements. At some threshold heating power being applied to the heating elements, the read/write elements will contact the surface of the magnetic disk. It is desirable to precisely determine the threshold heating power which caused the contact so that slightly lower heating powers may be used to obtain desired spacing between the read/write elements and the magnetic disk during normal operation while avoiding contact with the magnetic disk.
There are a variety of methods of detecting contact between the read/write elements and the magnetic disk so that the threshold heating power may be determined. One method comprises detecting a mechanical vibration in the recording head responsive to the contact between the read/write elements and the magnetic disk. Another method comprises monitoring the motor current being applied to the spindle motor, as the motor current should increase responsive to contact between the read/write elements and the magnetic disk in order to maintain a constant rotational speed. Another method comprises monitoring a servo timing signal read from the servo fields on the magnetic disk, as the servo timing signal should indicate a reduced rotational speed of the magnetic disk responsive to contact between the read/write elements and the magnetic disk. Yet another method comprises monitoring the VCM current to determine if there was a sudden radial movement of the recording head indicating contact with the magnetic disk.
Unfortunately, most of the present methods of detecting contact between the read/write elements and the magnetic disk may not be precise enough as the signals generated in these detection methods tend to have low signal-to-noise ratios (SNR). As a result, the threshold heating power identified as causing the contact between the read/write elements and the magnetic disk may not have the precision desired by disk drive manufacturers. Many interfaces in current disk drives have very little or no bouncing vibrations when at contact. However, every contact causes friction which in turn leads to a slowdown of the RPM. Hence friction based contact detection is more reliable than bounce based detection in most products.
Delayed contact detection caused by poor contact sensitivity may lead to excessive wear and burnishing of the read/write elements which in turn may lead to corrosion and interface failure.